I don't really know enough about John Gardner to be too confident in
my analysis what he intended by this book, and the online reviews and essays
are wildly contradictory. But personally I read it as a tale of Western
Christian triumphalism over existentialism. Beowulf and the other
humans, most importantly the Shaper, represent the idea that there is a
purpose to life and order in the universe. Grendel, influenced by
the dragon, believes that existence is its own end, that life is meaningless.
But even Grendel is plagued by the realization that his world view is empty
and unfulfilling, so when he hears the Shaper's song:

He told of an ancient feud between two brothers which
split the world between darkness and light.
And I, Grendel, was the dark side, he said in effect.
The terrible race God cursed. I believed him.
Such was the power of the Shaper's harp!

The Shaper's implicit message--that human existence serves a purpose
and that man, through the cultivation of art, science and religion, is
realizing a destiny--is more than Grendel can take:

Thus I fled, ridiculous hairy creature torn apart
by poetry... I gnashed my teeth and clutched the
sides of my head as if to heal the split, but I
couldn't.

Unable to reconcile the beauty of the Shaper's vision and his own dark
urge to deny existence any meaning, Grendel tries to destroy the humans.
But, of course, the humans in following the vision have developed the capacity
to withstand his primal fury and Beowulf dispatches him--thus ever the
confrontations between light and dark. To this extent, except for
giving the beast credit for a pretty well developed philosophical sense,
the story is relatively traditional and follows the original (read Orrin's
review). Moreover, it confirms our understanding of human history,
that great societies developed following the adoption of structured belief
systems, and that they overwhelmed those necessarily primitive groups which
failed to comprehend a general purpose to their own existence.

The great innovation here, and the malignancy within the novel, lies
in its narrative form. Gardner, by allowing the monster to narrate
the story, implies that there are two equally valid sides to the story.
This device has, not surprisingly, become quite common, particularly as
a way to let female and minority characters give their own angles on great
literature or history. Typically it is used to demonstrate that the
generally white, male, and Christian authors of the classics have presented
only a partial, perhaps propagandistic, and possibly simply dishonest,
version of events that actually happened quite differently. This
trend is part of the broader modern tendency toward moral relativism, political
correctness, and denial of absolute standards of morality, truth, and beauty.
In this instance, Gardner came down on the right side of the cultural divide,
and merely used the device to flesh out the monster. But as a general
proposition, the idea that the Western Canon presents only a partisan view
of the world, one that we need not assume is valid, is truly destructive
of our shared cultural inheritance and is to be abhorred. Stories
do indeed have two sides; but at the point where we surrender our capacity
to say that one side is right and one is simply wrong, we will have wrought
catastrophic damage upon our own culture..

That said, the book, though far inferior to the original, is still entertaining.

David Sandberg's Review:

How to explain the work Grendel? It's not easy. In my opinion
it is certainly one of the best works in English. But it is elusive to
explain exactly why this is so. Grendel manages to capture something
wonderfully, elegantly elusive in the human soul - something which does
not even truly have a name. I could write whole paragraphs simply trying
to elucidate on this ephemeral yet pervasive quality in the book - and
never really hit on it exactly. Simpler blunter words come easily to mind.
World angst. Existential futility. Ennui. None of these really do it for
me though.

Grendel is art. The plot of the book is, as most everyone knows,
the tale of Beowulf, which is habitually inflicted on High School
students as some kind of literary puberty rite in towns all across America.
I can't imagine how many millions of American school children have had
to delve into Beowulf, as College Prep students do, beginning their
obligatory peregrination into the origins of English. I will not discuss
Beowulf.
Having to read some of its passages in High School in Dedham Massachusetts
I have tried very successfully to block it out. Like Algebra,
Beowulf
has been meaningless in my life. Grendel tells the same tale from
the monster's point of view. Grendel has not been meaningless in
my life. Just the opposite.

I first read Grendel fifteen or twenty years ago. As anyone
who reads history knows - kingdoms, satrapies, empires, commonwealths,
archduchies, monarchies, nomates, ruled by kings, emperors, czars,
dauphins, viceroys, Grand Dukes et al come and go across the stage of human
history. They have their exits and their entrances and each plays its little
part in the progress of humanity. Marie Antoinette is interchangeable with
the Empress Alexandra who is interchangeable with Catherine de Medici and
so on. Kings and Queens come and go but the stories are chronically the
same. In Grendel we are exposed to this endless cyclicality of human
history. Grendel is only too aware of the futility of human progress. Times
change but people do not. They are small frail frightened creatures of
an hour. They live and die so easily. Ultimately they mean nothing. The
whole pageant of history sweeps on to another time and another place -
with the same cast of characters. For Grendel, humanity is utterly
boring - a dull thing.

He explores how the individual fits in the unfeeling insensitive universe.
He questions and dismisses God - he embraces nihilism and greed. He is
tutored in this by a Dragon. What is the individual? What is his place
in the scheme of things? What is a monster and what is a hero? Grendel
poses the questions. Grendel frames them for us with ease. But there are
no answers because the questions themselves are totally and purely irrational.
The universe, despite its fairy tale prettiness, is an empty directionless
thing. Simply conceiving of these soul testing questions is madness, and
Grendel is mad. How can a monster be anything else?